The present invention relates to a method for topographically mapping the surface of the eartH under utilization of measuring the altitude by means of radar, and under further utilization of a synthetic aperture for the radar, as well as an electronically controlled antenna lobe.
Topographically mapping the surface of the earth by means of scanning, under further utilization of radar-grammetry, while using satellite carried microwave sensors, is a technology which is just in its infancy stages. An impressive example for the potential power of such sensor systems was demonstrated by the Jet Propulsion Laboratory in Pasadena, Calif. Here, it was shown that through stereoscopic evaluation of respective two sequences of images, one could provide a narrow grid, digital altitude model of Mt. Shasta in Northern California. These images were provided by the shuttle imaging radar SIR-B program and resulted from several orbits. The results of this operation were published by F. W. Leberl et al, under the heading of "Radar Stereo Mapping Techniques and Application to SIR-B Images of Mt. Shasta", in IEEE Transaction Geoscience and Remote Sensing, Vol. GE 24, No. 4, of July 1986.
Several methods for areal coverage of radar altitude measurements were published in "Microwave Remote Sensing", Vol. 2, by F. T. Ulaby, R. K. Moore, and A. E. Fung. In this latter publication, the principle of generating a synthetic aperture of a radar beam (SAR) plays an important part, because this method requires a high areal resolution for a narrow grid altitude model which is equally applicable for measurements from outer space. Topographic methods, through evaluation of the parallactic effect, is made possible owing to the effect that results from two SAR images, taken from different orbital paths. Radar stereo mapping techniques and applications to SIR-B images of Mt. Shasta describe limited areal coverage and extremely expensive data acquisition. Therefore, this method is not really of great interest for world-wide topographic mapping.
A side-view radar, based on interferometry with a synthetic aperture, provides the desired topographic data directly from the measured angles of deflection and displacement of the image elements. An SAR system is described in "Microwave Remote Sensing" which requires two antennas and two image processors with relatively complex data channels related to amplitude and phase processing. The altitude measurements, however, exhibit ambiguities owing to the phase difference which results from integral multiple of +- pie. In order to obtain digital models of altitude, one will normally require only a moderate areal resolution such as 250 m by 250 m. On the other hand, the altitude resolution should be in the meter range (at the most). Here then, one can think of the classical principle of radar altitude measurements wherein the areal resolution is the result of the real or of a synthetic antenna aperture, while the altitude resolution results from short pulsing. However, this method is practical only for very steep angles of incidence, and the antenna lobe has to be pivoted in a suitable manner so that, indeed, one obtains sufficiently wide strips parallel to the foot path of the space vehicle.
A method has been devised by the Jet Propulsion Laboratory and described in "A Scanning Radar Altimeter for Mapping Continental Topography", by T. H. Dixon, Jet Propulsion Publication 86-26, 2nd Spaceborne Imaging Radar Symposium, JPL April 1986. This method involves a side-view radar with synthetic aperture wherein the antenna lobe is electronically pivoted transversely to the direction of flight. The Nadir is excluded here only to an extremely strong back scatter of the radar signal.